It's remarkable what can be created from so little. It's all the more remarkable for the number of times I've told myself I cannot create something because I don't have time. In only two-and-a-half hours at the weekend, I helped create a world from scratch and played a one-off campaign in it. To be ultra clear: nothing was prepared beforehand. In fact, the only real rule in the game was you cannot prepare beforehand. Otherwise, in You Awaken in a Strange Place, a pen-and-paper RPG, everything is created on the spot.
You establish the entire game world and rules, collaboratively, at the beginning. Individually, the game-master asks you questions that will shape the world. One person thinks of a genre, one person thinks of a place, and one person thinks of an adjective about that place. We think up "a steamy steampunk library" - you can see how on-the-spot it is.